As part of NCI's program to obtain and develop new and better cancer chemotherapeutic agents, in vitro testing for cytotoxicity is carried out on materials from natural sources primarily, and also on selected synthetic materials. Standardized assays (NCI protocols) are employed using KB, P388, and L1210 cells. Some methodological research is also conducted on improving current assays or developing new in vitro screens. Five thousand tests (or equivalent) are performed per year.